It is known to polymerize one or more olefins by starting with a fluidization gas containing the olefin(s) to be polymerized, in a fluidized bed reactor, in which polymer particles being formed are kept in the fluidized state by means of the fluidization gas travelling in an upward stream. The fluidization gas leaving at the top of the fluidized bed reactor returns to the base of the latter by means of a recycling conduit equipped with a compressor and generally with a heat exchanger.
During a fluidized bed polymerization, it has been observed that small variations in the progress of the polymerization, resulting, for example, from unavoidable slight fluctuations in the quality of the catalyst or of the olefins used, can give rise to changes in the behaviour and the catalytic activity of the polymer particles being formed. These small variations can cause an unexpected increase in the quantity of heat released by the polymerization. This can give rise to the appearance of hot spots in the bed and the formation of agglomerates of molten polymer. It is generally thought that a large fraction of these agglomerates is formed in the regions of the fluidized bed which are poorly stirred, and in particular along the vertical wall of the reactor.
It is also known to withdraw polymer from a fluidized bed reactor. The withdrawing of the polymer is generally accompanied by a quantity of gas which can be a relatively large quantity.